1. Field of the Invention
The present invention relates to a color signal processing apparatus for a single-plate type color video camera having a complementary color filter or a color still video camera.
2. Related Background Art
Conventionally, in an apparatus of this type, color filters shown in FIG. 1A are attached to a solid-state image pickup element, and signal processing shown in FIG. 2 is performed, thus finally obtaining a luminance signal and two color difference signals R-Y and B-Y.
In conventional color signal processing, calculation processing is normally started from color difference signals as a result of subtracting outputs from horizontally adjacent pixels with different color filters. For example, if the color filter matrix shown in FIG. 1A is interlace-scanned in a frame storage mode, video signals obtained from a sensor 1 are subjected to .gamma. correction in a .gamma. correction circuit 2, and are then processed by a luminance signal processing circuit 3, thus extracting a luminance signal Y. In an odd-numbered column of each field, a subtraction result of C.sub.1 =(Mg-Gr) is obtained by a subtracter 4, and in an even-numbered column, a subtraction result of C.sub.2 =(Ye-Cy) is obtained. Meanwhile, a color signal processing circuit 5 performs color processing calculations for, e.g., white balance, .gamma. conversion, and the like by a proper method.
A coincidence circuit 6 causes these line-sequential color difference signals C.sub.1 /C.sub.2 to coincide with each other using a 1H (horizontal scanning period) delay line or the like. These signals are then input to a color difference matrix circuit 7, so that their color difference axes are appropriately rotated, thus finally obtaining two color difference signals R-Y and B-Y.
However, the color processing method of this type has the following fundamental problems.
(A) A white balance is difficult to obtain.
In a three-tube type camera or an RGB primary color (pure color) type camera, ratios of R and B to G are changed in accordance with a change in color temperature, thus obtaining a white balance. However, in the apparatus of this type, since color data is obtained in the form of a color difference, a fraction of a luminance signal is added/subtracted to/from a color difference signal in accordance with a color temperature to forcibly set a color difference signal for white to zero, thereby obtaining a white balance. With this method, it is very difficult to accurately obtain a white balance over a wide color temperature range.
(B) Since the color difference signals are directly subjected to .gamma.-conversion, color reproducibility is poor.
In a three-tube type camera or an RGB primary color type camera, outputs R, G, and B which are color-separated in accordance with an NTSC scheme are multiplied with .gamma. to obtain R.gamma., G.gamma., and B.gamma.. Thereafter, two color difference signals R.gamma.-Y and B.gamma.-Y are obtained. In this case, Y (luminance signal) is given by Y=0.30R.gamma.+0.59G.gamma.+0.11B.gamma..
In a complementary color type camera, however, since differences between color signals are first calculated and then multiplied with .gamma., they are multiplied with .gamma. in the form of a difference like (Mg-Gr).gamma.. Therefore, if any correction is made later, color signals having a correspondence with normal NTSC signals cannot be obtained, and color reproducibility is poor.
In order to solve the above problems, for example, as shown in FIG. 3, two color difference signals C.sub.1 and C.sub.2 obtained through a subtracter 8 and a coincidence circuit 9 are converted to R, G, and B signals using a luminance signal Y.sub.L ' passing through a low-pass filter 10 by an appropriate calculation in an RGB conversion unit 11. In this state, the R, G, and B signals are subjected to white balance processing and .gamma. conversion by a white balance circuit 12 and a .gamma. conversion unit 13, respectively. Thereafter, the R, G, and B signals are converted again to a luminance signal Y.sub.L and color difference signals R-Y and B-Y by a color difference matrix circuit 14.
With this method, since white balance processing and conversion can be performed in the form of R, G, and B signals, the above-mentioned problems in FIG. 2 can be eliminated. The luminance signal is extracted such that a high-frequency luminance signal Y.sub.H is extracted from the output of a sensor 1 using a high-pass filter 15, and a low-frequency luminance signal is added thereto by an adder 16.
However, in a method wherein after color difference signals of horizontal output differences are formed, and color processing is performed based on these signals, optimal color processing matching with spectral sensitivity of filters cannot be performed, and color reproducibility cannot be improved.